28.12

Ecological Succession

19.5K views02:17 min

Ecological succession is the gradual change in the structure and composition of a biological community over time. This process is often triggered by environmental disturbances.

Ecologists divide succession into two main types that follow a disturbance: primary succession and secondary succession.

Primary succession begins when living organisms colonize bare rock left behind by retreating glaciers or formed after volcanic eruptions. These environments contain no visible life and no developed soil.

The first organisms that establish under these extreme conditions are pioneer species, which can tolerate the harsh conditions created by environmental disturbances.

For example, lichens are common pioneer species. They are symbiotic associations between a fungus and an alga or a cyanobacterium.

The fungus attaches to the rock and absorbs minerals released during weathering, while the alga performs photosynthesis. Over time, lichens speed up rock breakdown by releasing acids and physically penetrating the surface.

As lichens grow and decompose, their organic remains combine with rock fragments to form a thin soil layer. As this soil deepens and becomes nutrient-rich, additional plant species establish in a predictable sequence.

Mosses often appear first, followed by grasses, then shrubs, shade-intolerant trees such as pines, and finally shade-tolerant trees such as oaks.

Over time, species composition becomes more stable and self-sustaining. This relatively stable stage is called a climax community. This state is not permanent, as community composition continues to shift in response to ongoing environmental variation. It persists until a new disturbance alters the environment. Primary succession may take hundreds of years to reach a climax community.

Secondary succession happens when a disturbance, such as a fire, removes existing vegetation but leaves the soil largely intact. Because the soil, seeds, and underground structures remain, recolonization happens more rapidly.

Pioneer species such as herbaceous perennials and grasses reinhabit the soil first, followed by trees. For example, oak and hickory forests often reach a stable climax community within about 150 years.

The climax community formed through secondary succession may differ from the original community.

Overview

Loading...

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

Ecological succession is influenced by the processes of facilitation, inhibition, and toleration. Facilitation occurs when early successional species create more favorable ecological conditions for subsequent species, such as enhanced nutrient, water, or light availability. In contrast, inhibition happens when early successional species create unfavorable ecological conditions for potential successive species, such as limiting resource availability. In some cases, later successional species only have the chance to thrive if a disturbance negatively impacts the early inhibitory species. Finally, toleration occurs when the ecological conditions created by early successional species neither aid nor impede the emergence of later succession species.

For example, ecologists have extensively studied primary succession resulting from glacier retreats at Glacier Bay in Alaska. Over a period of 1,500 years, pioneer species such as liverworts paved the way for creeping shrubs, which in turn set the stage for larger shrubs and trees like alder. Eventually, a climax community emerged that was dominated by spruce trees. Facilitation and inhibition influenced this succession pattern. Dryas shrubs and alders improved the nitrogen content of the soil, facilitating the establishment of spruce seedlings. However, competition and leaf litter produced by these early successional species also hindered later species’ germination and seedling survival.

Understanding ecological succession is important because humans significantly impact ecological communities. Agriculture, clear-cutting, and overgrazing by livestock disturb terrestrial ecosystems, causing species diversity to decline. Although ecosystems can naturally recover from such disturbances via ecological succession, severe damage (e.g., soil nutrient loss or toxic chemicals) may prolong or prevent recovery. To address this issue, restoration ecologists apply the principles of ecological succession to accelerate the time to climax community—thus, repairing the damaged ecosystem.

Transcript

Loading...

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

Ecological succession is the gradual change in the structure and composition of a biological community over time. This process is often triggered by environmental disturbances.

Ecologists divide succession into two main types that follow a disturbance: primary succession and secondary succession.

Primary succession begins when living organisms colonize bare rock left behind by retreating glaciers or formed after volcanic eruptions. These environments contain no visible life and no developed soil.

The first organisms that establish under these extreme conditions are pioneer species, which can tolerate the harsh conditions created by environmental disturbances.

For example, lichens are common pioneer species. They are symbiotic associations between a fungus and an alga or a cyanobacterium.

The fungus attaches to the rock and absorbs minerals released during weathering, while the alga performs photosynthesis. Over time, lichens speed up rock breakdown by releasing acids and physically penetrating the surface.

As lichens grow and decompose, their organic remains combine with rock fragments to form a thin soil layer. As this soil deepens and becomes nutrient-rich, additional plant species establish in a predictable sequence.

Mosses often appear first, followed by grasses, then shrubs, shade-intolerant trees such as pines, and finally shade-tolerant trees such as oaks.

Over time, species composition becomes more stable and self-sustaining. This relatively stable stage is called a climax community. This state is not permanent, as community composition continues to shift in response to ongoing environmental variation. It persists until a new disturbance alters the environment. Primary succession may take hundreds of years to reach a climax community.

Secondary succession happens when a disturbance, such as a fire, removes existing vegetation but leaves the soil largely intact. Because the soil, seeds, and underground structures remain, recolonization happens more rapidly.

Pioneer species such as herbaceous perennials and grasses reinhabit the soil first, followed by trees. For example, oak and hickory forests often reach a stable climax community within about 150 years.

The climax community formed through secondary succession may differ from the original community.

Explore More Videos

Ecological Succession

Primary Succession

Pioneer Species

Climax Community

Ecosystem Recovery

Restoration Ecology